Traditional analog voice telecommunication was carried over dedicated circuits, so that each connection used the full bandwidth of a circuit. With the advent of digital telecommunications, data and voice messages became packetized, so that a physical circuit may carry multiple virtual circuits. While this provided the advantages of greater efficiency in terms of more usage of each physical circuit, the tradeoff was occasional quality issues, such as latency if a particular circuit was over utilized.
Synchronous Digital Hierarchy (SDH) and synchronous optical network (SONET) were originally developed to transport voice and data over dedicated fiber optic cables. Several methods exist for transmitting SDH/SONET signals over packet networks using pseudo wires (PW), such as Circuit Emulation over Packet (CEP) as defined in RFC 4852.
Among the many protection mechanisms in available SDH and SONET technologies, Sub-network Connection Protocol (SNCP) and Unidirectional Path Switched Ring (UPSR) can be used to provide path protection. In these protection schemes, a SNCP termination equipment (STE) transmits two copies (working and protection) of the protected path over typically disjoint routes, while the STE at the receiving end switches from the working to the protection path when the working path fails or its performance falls below the required level.
SNCP provides protection in the sub-network level, so when a path route includes multiple sub-networks, failures in an individual sub-network are locally repaired and do not affect protection offered at the other sub-networks. A typical example of sub-networks is the SDH ring. UPSR is defined for a ring topology only, and the same principle applies with rings instead of sub-networks.
To provide protection against a node failure, sub-network interconnection can take place across two different nodes. In this case, the connection at one location is connected to the network using two network connections, for example, an add-drop multiplexer. The first node may be active, while the second node may be standby. If the active node fails, for example due to a hardware failure, the standby node becomes active, minimizing the loss of network connection time. This is known as dual node interconnection (DNI) or dual homing.
Using DNI in SNCP applications may cause additional complexity in the protection scheme, as SNCP and UPSR provide protection against a single failure in each sub-network, and DNI also protects against interconnecting failures. By repairing failures in the sub-network level, a failure in one sub-network may not be contained within the sub-network, thus impacting protection in other sub-networks.
FIG. 1 is a schematic diagram of a prior art first network 100 having four nodes 110, including a first node 110A, a second node 110B, a third node 110C and a fourth node 110D. Each node 110 may be, but is not limited to, for example, path termination equipment (PTE). The nodes 110 are configured such that the first node 110A and the second node 110B form a DNI setup on a local side of an MPLS sub-network 150, and the third node 110C and the fourth node 110D form a DNI setup on the remote side of the MPLS sub-network 150. The first node 110A and the third node 110C are connected by working connection PW-13, and the second node 110B and the fourth node 110D are connected by protection connection PW-24. A failure in, for example PW-13 will not be contained to the MPLS sub-network, since the signal received by the first node 110A will not be transmitted by the third node 110C, so a failure in the working PW in a SDH sub-network connected to the third node 110C and the fourth node 110D would result in a complete failure in the protected PW. In this case, a failure in the MPLS sub-network affects the level of protection in the interconnected SDH sub-network, such that the protection scheme does not protect against a single failure in each sub-network. Therefore, there is an unmet need to implement SNCP/UPSR over packet switched networks using DNI, as well as single node interconnections.